六方氮化硼：量子發(fā)射器的輻射特性

二維材料作為一個可以應(yīng)用于光量子信息技術(shù)的載體，近年來得到快速發(fā)展。其中，六方氮化硼中基于點缺陷的單光子發(fā)射體的發(fā)現(xiàn)，激發(fā)了人們對相關(guān)材料中光學(xué)活性缺陷的大量研究。六方氮化硼的缺陷發(fā)射器目前面臨的挑戰(zhàn)是如何確定它們的原子結(jié)構(gòu)。基于密度泛函理論的第一性原理計算能夠提出各種可能的缺陷結(jié)構(gòu)，且能與實驗對比，但不能處理單光子發(fā)射器的一些關(guān)鍵特性，如激發(fā)態(tài)和輻射過程?；诹孔踊瘜W(xué)和密度矩陣重整化組等方法也已被用于研究特定的缺陷結(jié)構(gòu)，但迄今很少有報道將不同結(jié)構(gòu)進行對比。在本工作中，來自美國加州理工學(xué)院應(yīng)用物理與材料科學(xué)系的Marco Bernardi教授等人采用密度泛函理論和GW+Bethe-Salpeter方程的計算方法，研究了六方氮化硼中原子缺陷的光學(xué)性質(zhì)、躍遷偶極子、激子和輻射壽命等性質(zhì)。研究發(fā)現(xiàn)，不同的量子發(fā)射器的輻射壽命橫跨6個數(shù)量級，發(fā)射能量涉及紅外到紫外光區(qū)。通過貝葉斯統(tǒng)計分析，他們確認(rèn)了各種高度可能的電荷中性缺陷發(fā)射體，其中本征缺陷VNNB的發(fā)射能量和輻射壽命與實驗數(shù)據(jù)一致。通過對VNNB缺陷的進一步計算，作者發(fā)現(xiàn)其輻射特性對其原子結(jié)構(gòu)的小擾動有很強的依賴性。該工作推進了對基于六方氮化硼單光子發(fā)射體的微觀理解，并引入了一個計算框架來表征和識別二維材料中的量子發(fā)射體。該文近期發(fā)表于npj Computational Materials7: 85 (2021)。

Fig. 2 Radiative lifetime and energy of the lowest bright exciton of candidate defect SPEs in hBN from GW-BSE calculations.  

Editorial Summary

Quantum emitters in hBN: Radiative properties As a platform for photon based quantum information technologies, two-dimensional (2D) materials have been growing rapidly in recent years. The discovery of single-photon emitters (SPE) at point defect in hBN has spurred an intense search for optically active defects in 2D materials. Currently, a pressing challenge for defect emitters in hBN is identifying their atomic structure. Density functional theory (DFT) based first-principles calculations are able to identify various possible structures that can be compared to experiments, but DFT cannot address key aspects of point-defect SPEs such as their excited states and radiative processes. Quantum chemistry approaches and density matrix renormalization group have also been used to investigate specific defect structures, but wide comparisons among different structures are still missing. In this work, Prof. Marco Bernardi from the Department of Applied Physics and Material Science, California Institute of Technology, used ab initio density functional theory and GW plus Bethe-Salpeter equation calculations to investigate the optical properties, transition dipoles, excitons and radiative lifetimes of atomic defects in hBN. They found that different quantum emitters exhibit radiative lifetimes spanning six orders of magnitude and emission energies from infrared to ultraviolet. Through the Bayesian statistical analysis, they identified various high-likelihood charge-neutral defect emitters, among which the native VNNB defect possesses emission energy and radiative lifetime in agreement with experiments. In-depth calculations on the VNNB defect highlight the strong dependence of radiative properties on small perturbations to the atomic structure. This work advances the microscopic understanding of hBN single-photon emitters and introduces a computational framework to characterize and identify quantum emitters in 2D materials. This article was recently published in npj Computational Materials 7: 85 (2021).

Fig. 3 Bayesian inference workflflow.

原文Abstract及其翻譯

Radiative properties of quantum emitters in boron nitride from excited state calculations and Bayesian analysis (氮化硼中量子發(fā)射體輻射特性的激發(fā)態(tài)計算和貝葉斯分析)

Shiyuan Gao, Hsiao-Yi Chen and Marco Bernardi

Abstract Point defects in hexagonal boron nitride (hBN) have attracted growing attention as bright single-photon emitters. However, understanding of their atomic structure and radiative properties remains incomplete. Here we study the excited states and radiative lifetimes of over 20 native defects and carbon or oxygen impurities in hBN using ab initio density functional theory and GW plus Bethe-Salpeter equation calculations, generating a large data set of their emission energy, polarization and lifetime. We find a wide variability across quantum emitters, with exciton energies ranging from 0.3 to 4 eV and radiative lifetimes from ns to ms for different defect structures. Through a Bayesian statistical analysis, we identify various high-likelihood charge-neutral defect emitters, among which the native VNNB defect is predicted to possess emission energy and radiative lifetime in agreement with experiments. Our work advances the microscopic understanding of hBN single-photon emitters and introduces a computational framework to characterize and identify quantum emitters in 2D materials.

Fig. 4 Properties of the VNNB defect.

摘要六方氮化硼(hBN)中的點缺陷作為單光子發(fā)射體越來越受到人們的關(guān)注。然而，目前對它們的原子結(jié)構(gòu)和輻射特性的理解仍然不完整。在這里，我們使用從頭算密度泛函理論和GW+Bethe-Salpeter方程的計算方法，研究了hBN中20多種本征缺陷和碳或氧雜質(zhì)缺陷的激發(fā)態(tài)和輻射壽命，給出了它們的一個包含發(fā)射能量、極化和壽命的大數(shù)據(jù)集。我們發(fā)現(xiàn)，量子發(fā)射器的可變性很大，對于不同的缺陷結(jié)構(gòu)，激子能量在0.3至4 eV范圍內(nèi)，輻射壽命在ns 至ms范圍內(nèi)。通過貝葉斯統(tǒng)計分析，我們確定了各種高度可能的電荷中性缺陷發(fā)射體，其中對本征VNNB缺陷預(yù)測的發(fā)射能量和輻射壽命與實驗數(shù)據(jù)一致。我們的工作推進了對hBN單光子發(fā)射器的微觀理解，并引入了一個計算框架來表征和識別二維材料中的量子發(fā)射體。